1. Field of the Invention
The present invention relates to a spin coating process and, more particularly, to a spin coating process for preventing a coating composition applied to the surface of a base plate from adhering to the back side thereof and to the edge or periphery of the plate.
2. Description of the Prior Art
A technique of forming a photoresist on a base plate by spin coating using an organic solvent as the solvent for the coating composition has widely been employed in the field of producing semiconductor devices, etc. For example, a photoresist coating layer of a uniform thickness is formed by placing a base plate, such as a glass sheet having thereon a thin metallic layer, a silicon wafer, etc., on a turntable of a spinner, applying dropwise a coating composition for a photoresist onto the turntable, and then rotating the turntable at high speed (usually at about 2,000 to about 6,000 r.p.m.).
However, attempts to coat a gelatino silver halide emulsion on a base plate such as a glass sheet and a glass sheet having thereon a thin metal layer by spin coating involve the following difficulties. That is, since the solids content of a gelatino silver halide emulsion is much lower than that of a photoresist coating composition, the dry thickness of the silver halide emulsion layer formed is much thinner than the thickness of the silver halide emulsion layer immediately after coating, namely, the dry thickness of the silver halide emulsion layer is usually about 1/50th the thickness of the silver halide emulsion layer immediately after coating. Therefore, if a gelatino silver halide emulsion is coated by spin coating at high speed as in the case of coating a photoresist coating composition, the dry thickness of the silver halide emulsion layer formed is extremely thin (for example, about 1/2 to about 1/10 micron). Also, when a gelatino silver halide emulsion is coated by spin coating, the emulsion layer formed contains a large quantity of bubble traces, similar in appearance to a comet, since fine bubbles enter the silver halide emulsion when it spreads over the base plate by the action of centrifugal force.
Furthermore, when the temperature of a gelatino silver halide emulsion layer falls to about room temperature (e.g., about 20.degree.-30.degree. C.), the emulsion layer is set, but when the speed of rotation in spin coating is high, the emulsion is frequently set while the emulsion is being spread over a base plate by the rotation of the turntable. On the other hand, the periphery or edge of the set coating is subjected to a large centrifugal force and thus, the emulsion layer at the periphery is sometimes spun off. In this case, it would be desirable for the coating composition to be uniformly spun off at the periphery to expose the surface of the base plate at the periphery in a uniform width but the width or the amount of the emulsion layer thus spun off varies greatly at each position or area. However, when the solids content of a gelatino silver halide emulsion is increased to overcome these difficulties, the silver halide emulsion has an even greater tendency to set, which is undesirable.
Therefore, when a silver halide emulsion is coated on a base plate by spin coating, the coating must be performed at much lower speed of rotation about 100 to about 1,000 r.p.m.) than that used in the above-described photoresist coating. Furthermore, when the viscosity of a coating composition is low or the solids content of a coating composition is low in the case of coating a photoresist coating composition on a base plate or when a coating layer having a thickness of about 5 to about 20 microns is required as in the case of coating a photoresist coating composition of, for example, a copper laminate for a printed circuit, the spin coating must be carried out at a low speed of rotation.
However, when spin coating is performed by rotating the base plate at such a low speed of rotation (in particular, at a low speed of rotation of about 100 to about 500 r.p.m.), the coating composition accumulates at the periphery and further on the back surface of the plate. This phenomenon occurs more markedly at the periphery or the back surface at the corner portions when the base plate has a square or rectangular corner. If the coating composition adheres to the periphery or the back surface of the base plate as mentioned above, the coating composition thus adhered is scraped off after drying as fine pieces, which attach to the coated layer on the surface of the base plate to cause defects such as pinholes. Prevention of this problem is very important if fine images such as those required in production of semiconductor devices, are to be obtained.